1. Density is the ratio of the mass of a substance or object to its volume. At any given temperature, the ratio of mass to volume is characteristic for a given substance.
a) The density of the liquids does not effect on the pressure. But the temperature effects the density of liquids. An increase in temperature produces an increase in the kinetic energy available to the molecules, enabling them overcome the intermolecular forces to a greater extent and move further apart. This behavior decreases the density of the liquid.
b) Liquids cannot be compressed easily. So if you wanna observe changing density of a liquid you need a great pressure.
c) The pressure of the gas in increased by decreasing the volume the gas may occupy (thereby producing more molecule-wall collisions and thus a higher pressure). The molecules are forced to occupy a much smaller space, producing a higher density.